With increased concerns of drought, we must understand how weeds will respond to drought because weeds are the number one biotic limitation to crop yields among all the pests; and weeds are known for their ability to adapt to abiotic stresses compared with most crops. Several studies over the years have evaluated weed responses to drought; however, this impact has not been systematically recorded and synthesized across diverse studies.

To address this knowledge gap, a global meta-analysis was conducted using 1,196 paired observations from 86 published articles assessing the effect of water stress on weed germination, growth characteristics and seed production. These studies were conducted and published during 1970-2020 across four continents (Asia, Australia, Europe and North America). A multi-step screening protocol was adopted to identify relevant literature for this meta-analysis (Figure 1). Imposed water stress was expressed as solution osmotic potential (ψ_solution), soil water potential (ψ_soil), or soil moisture as percent field capacity (Singh et al. 2022).

Figure 1. PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) flow diagram highlighting the selection procedure of 86 scientific published papers included in the meta-analysis.

Meta-analysis is a powerful statistical technique to quantitatively synthesize results from published papers on certain topic. Using this technique, we collected and analyzed results from 86 studies worldwide investigating weed responses to drought. A specific criterion was defined to include relevant studies in meta-analysis:

1. Water-stressed and well-watered (control) treatments should be tested under the same experimental conditions, and water stress should be maintained for the whole duration of the experiment.
2. Water stress should be expressed either as “solution osmotic potential”, “soil water potential” or “percent field capacity”.
3. Weeds should be grown separately from crops as monoculture, and data should be collected for at least one response variable associated with either weed germination, growth characteristics or seed production.

Overall Findings

1. The germination of grass weeds showed relatively greater inhibition than broadleaf weeds at a majority of the water stress levels, but there was no significant difference between them. To reduce germination by half, a minimum of -0.32 MPa solution osmotic potential was required for broadleaf weeds, while only -0.09 MPa was required for grass weeds. However, solution osmotic potential above -0.8 MPa completely inhibited the germination of both broadleaf and grass weeds.
2. The negative effect of drought increased with increasing level of water stress intensity (Figure 2). For example, belowground weed growth as characterized by root biomass decreased by 39% at moderate water stress (30-60% field capacity), whereas it was reduced by 69% at severe water stress (<30% field capacity). Similarly, above-ground weed characteristics such as plant height, leaf area and shoot biomass were decreased by 24%, 43% and 39% at moderate water stress (30-60% field capacity) to 37%, 44% and 61% at severe water stress (<30% field capacity), respectively.
3. Weeds allocate higher portions of dry matter to roots under water stress. This was evident from an overall 19% increase in root:shoot ratio due to water stress. It indicates that plants (weeds, in our case) under water stress prioritize root growth to increase rooting depth, which allows them to extract water from deeper layers and maintain higher water influx to roots for longer periods.
4. Even when water stressed, weeds continue to produce seeds. For example, seed per plant was decreased by 50% under moderate water stress (30-60% field capacity), and by 88% under severe water stress (<30% field capacity). Seed production was not completely inhibited even under severe water stress.
5. Majority of the studies in meta-analysis expressed water stress as “solution osmotic potential” and were conducted in Petri dishes (germination studies) or in greenhouse (growth or seed production studies). This may not truly represent seed-soil-water interactions as prevalent in fields. Therefore, more field-based studies expressing water stress as “percent field capacity” or “soil water potential” should be conducted to get more accurate representation of water stress effects on weeds.

Figure 2. The percent change in response of weed growth characteristics (plant height, leaf area, branches/tillers per plant, leaves per plant, root biomass, shoot biomass, and root:shoot ratio) and seed production (inflorescences per plant and seeds per plant) to different levels of water stress. Water stress decreased as soil moisture (% field capacity) increased and vice versa. The green and red dots represent broadleaf and grass weed species, respectively. The solid black points and the lines represent the mean percent change in response and their 99% confidence intervals (CIs) for low (>60%), moderate (30%–60%), and severe (<30% field capacity) water-stress subgroups.

What Does This Mean For Practical Weed Management? 

1. We cannot ignore the main findings that weeds will continue to germinate, grow, survive and produce seeds even under drought conditions. This implies that weeds will continue to be competitive, problematic and troublesome during drought years.
2. As science suggests that cropping systems are expected to experience extreme weather such as drought at higher frequencies than ever before, we should aim to direct our future research efforts to investigate and encourage the use of diverse multiple strategies as integrated weed management plan to effectively manage weeds under water-limited environment.
3. Scouting for weeds is important in the fields that are under drought, and make a plan for applying labeled post-emergence herbicides. Remember that weeds under drought stress can respond less to herbicides; therefore, wait for some rain and apply herbicide when weeds are healthy and actively growing.